Ser Jonothor was one of Prince Rhaegar Targaryen's three Kingsguard who rode with him from the Red Keep to lead the royal army at the battle of the Trident.[5] Before departing, Jaime pleaded with Rhaegar to command Jon to remain at the Red Keep and guard Aerys in his stead. In response, Jon snapped at his younger sworn brother and reminded him of his duty.

Ser Jon was slain at the Trident,[5] cut down in the midst of the conflict.[6]